The background is described with respect to LTE (Long Term Evolution). The skilled person will however realize that the principles of the invention may be applied in other radio communication systems, particularly in communication systems that rely on scheduled data transmissions.
The downlink transmission of the LTE (Long Term Evolution), or E-UTRAN radio access, is based on Orthogonal Frequency Division Multiplex (OFDM). The basic LTE downlink physical resource can thus be seen as a time-frequency grid as illustrated in FIG. 1, where each resource element (RE) corresponds to one OFDM subcarrier during one OFDM symbol interval. The dark shadowed resource elements form a resource block.
In the time domain, transmissions in LTE are structured into frames and subframes. Each frame of length Tf=10 ms consists of ten equally-sized subframes of length Tsubframe=1 ms. Each subframe, in turn, consists of two equally-sized slots of length Tslot=0.5 ms.
Resource blocks (RBs) are also defined in LTE, where each RB consists of 12 contiguous subcarriers during one slot. The subcarrier spacing is set to Δf=15 kHz. In addition, a reduced subcarrier spacing of 7.5 kHz is defined targeting multicast broadcast transmissions in single-frequency networks.
Generally a resource element may be defined by certain ranges in any combination of the transmission resource, which are essentially time, frequency, code and space, depending on the actual transmission system under consideration.
The LTE time domain structure, in which one radio frame is divided into the 10 subframes #0 to #9 and each subframe is divided into a first and a second slot as depicted in FIG. 2.
In LTE data transmissions to/from a user equipment (UE) are under strict control of the scheduler located in the eNB. Control signaling is sent from the scheduler to the UE to inform the UE about the scheduling decisions. This control signaling, consisting of one or several PDCCHs (Physical Downlink Control Channels) as well as other control channels, is transmitted at the beginning of each subframe in LTE, using 1-3 OFDM symbols out of the 14 available in a subframe (for normal CP and bandwidths larger than 1.8 MHz, for other configurations the numbers may be different).
Downlink scheduling assignments, used to indicate to a UE that it should receive data from the eNB occur in the same subframe as the data itself. Uplink scheduling grants, used to inform the UE that it should transmit in the uplink occur a couple of subframes prior to the actual uplink transmission.
Generally, control data may comprise at least one of a downlink assignment and an uplink grant.
Among other information necessary for the data transmission, the scheduling assignments (and grants) contain information about the frequency-domain location of the resource blocks used for data transmission in the first slot. The frequency-domain location of the RBs in the second slot is derived from the location in the first slot, e.g. by using the same frequency location in both slots.
Thus, scheduling assignments/grants operate on pairs of resource block in the time domain. An example hereof is shown in FIG. 3.
In FIG. 3, the slopingly hatched parts in each resource block 0 to 9 contains control data, whereas the horizontally hatched parts contain payload data. The subframe is divided into a first slot and a second slot. The control data is part of the first slot.
For LTE Release-11 an enhanced Physical Downlink Control Channel is being discussed. In the following it is referred to as ePDCCH. This control channel is used to transmit control data/control signaling. Investigations are motivated by RAN1 email discussion “[66-04] Downlink control signalling enhancements”:
Firstly, the PDCCH (Physical Downlink Control Channel) does not provide the flexibility in frequency domain for control channel interference coordination between cells or any potential for frequency selective scheduling gain of the control channel. Secondly, the PDCCH overhead does not scale well with the number of scheduled UEs. Thirdly, the growing use of PDSCH (Physical Downlink Shared Channel) transmission in MBSFN (Multicast/Broadcast Single Frequency Network) subframes is limited by the fact that only two OFDM symbols can be used for PDCCH. Fourthly, the PDCCH cannot leverage advantage of multiple antennas at the eNB through beamforming gain to make the control channel performance scale with the number of transmit antennas. And last, the intra-cell spatial reuse of control channel resources which is useful for novel deployments and antenna structures as the shared cell scenario is not possible with the PDCCH.
Multiple ePDCCHs can be transmitted in a subframe and similar to the LTE 3GPP Release 8 PDCCH the concept of a search space will be applied: a search space is a set of locations in the time-frequency grid, where the UE (or any receiving node of the control channel) can expect an ePDCCH transmission. The Release 8 control region (i.e. the region, wherein the search space is determined) spans the whole freq. domain and the search space is determined taking all RBs into account. The ePDCCH control region will typically not occupy the full system bandwidth so that the remaining resources can be used for other kinds of transmission, e.g., data to UE.
The invention is particularly relevant for LTE based systems. Downlink control signaling is discussed in Section 16.2.4, pages 333 to 336, of the book entitled 3G Evolution: HSPA and LTE for Mobile Broadband, first edition 2007 by Dahlmann, Parkvall Skoeld and Beming. It is also pointed to the standards 3GPP LTE Rel-10. The cited references/documents are incorporated by reference herewith.